1. Field of the Invention
This invention relates generally to magnetic recording disks and, more particularly, to preconditioning such disks prior to use thereof.
2. Background Information
Disks used for magnetic recording and storage of data have a number of layers that perform various functions for correctly writing data on the disk, reading data back from the disk, and locating data in the appropriate track on the disk. The composition of the layers of a disk depends upon whether the disk is used for longitudinal recording or perpendicular magnetic recording, for example. The initial magnetization of the layers must be controlled and this differs depending upon the composition of the layers and the intended use of the disk.
Recently, the trend in magnetic recording is to employ perpendicular magnet recording techniques. In perpendicular magnetic recording, the magnetic domains are aligned perpendicularly to the surface of the disk platter. This allows bits to be placed closer together on the platter, thus increasing storage density over that which has been achieved in conventional longitudinal recording in which the domains are in the plane of the disk.
In a perpendicular magnetic recording disk medium, a first layer is typically a mechanical protective layer comprised primarily of carbon. This layer protects the next layer, which is the magnetic layer where the data is stored. An intermediate layer is often used to aid in obtaining perpendicular orientation of the magnetic moments in the media magnetic layer.
Perpendicular magnetic recording disks also employ a soft magnetic underlayer (also referred to as the “SUL”), which serves as a conduit for the magnetic recording flux. During a write operation, the magnetic flux emanates from a monopole writing element, and is directed through the recording layer and returns through the soft under-layer path and back to the writing element. Thus, it is desirable that the soft magnetic underlayer exhibit low coercivity, high saturation magnetization, and moderate but constant permeability in the range of the write fields.
However, demagnetizing fields of the outer and inner edges of a magnetic disk give rise to the formation of complex closure domain structures known as “domain walls” which can interfere with the storage medium flux and result in a phenomenon as “spike noise.” Specifically, domain walls form in a soft underlayer between adjacent regions that are magnetized radially outwardly and radially inwardly. In between the two regions, the magnetization must cross the domain wall, that is, change direction, which causes the magnetic flux to rotate outwardly in a perpendicular orientation and then back down to the plane of the disk. While the flux is pointed generally perpendicular to the disk, the magnetic flux creates a spike, (commonly known as “spike noise”). The spike noise can reach the read head and cause errors. The spike can be approximately 1 micron wide, and can thus affect about 40 bits of data, possibly creating read errors in those 40 bits of data.
In order to address this, many disks employ a hard bias layer adjacent to the soft underlayer material. This hard bias layer is a magnetic material which is coupled to the soft underlayer material to cause a radial biasing of the SUL. The hard bias layer must be initially set so that it properly biases the SUL radially, i.e., in the plane of the disk.
A technique had been proposed for radially magnetizing the hard bias layer of a SUL using a large permanent magnet to precondition the disk. Using this technique, the disk is passed through the permanent magnet such that the disk is subjected to an in plane field. When the process is completed, the disk is removed from the magnet. During removal, the associated edge fields continue to affect the disk as the disk is being removed. These edge fields have perpendicular components that thus interfere with the desired radial pattern of magnetization.
In addition to presetting the hard bias layer of the SUL, the data layer of the disk must be initially set such that it is in the equivalent of an AC demagnetization state. This provides an optimum condition for the data to be written onto the disk successfully and reliably. Perpendicular components are preferably avoided in setting the data layer because such components interfere with proper write and read back operations.
In prior techniques, particularly in longitudinal recording, a DC erase process was used to preset the magnetization of the data layer. However, in perpendicular magnetic recording media, a DC erase process is disadvantageous due to the density of the cells in which the magnetic domains are contained. It is particularly harmful in the servo regions. Thus, a DC erase process is not suitable for preconditioning a disk which is for use in perpendicular magnetic recording.
Furthermore, the data layer cannot be set by a permanent magnet in the same way as the hard bias layer in prior techniques because the field required for data layer setting is larger than that needed to set the hard bias layer. More specifically, the field required to completely wipe clean and preset the data layer is on the order of 9,000 Oersteds (Oe). Such a large field is typically not capable of being produced by a large permanent magnet of a manageable size.
Moreover, a large permanent magnet has a perpendicular field component at the exit that leaves the data layer with some perpendicular DC magnetization, which can cause asymmetry problems during spiral self servo write. In addition, some fields produced by large permanent magnets can be non-uniform, which is also undesirable in this context. In addition, such large permanent magnets are quite expensive and may require massive shielding to protect other equipment in the vicinity.
To reset the data layer, it has been proposed to sweep an in-plane permanent magnet from the inner diameter (ID) to the outer diameter (OD) of a revolving disk on a certifier tool. However, this certifier tool technique tends to leave residual magnetization in the data layer in a spiral pattern that can also interfere with spiral self servo write.
There remains a need, therefore, for an apparatus and method for placing the data layer of a disk quickly into the equivalent of an AC demagnetization state, and which apparatus and method can also be used to preset the hard bias layer of the SUL to produce an essentially domain wall free SUL, with a hard bias layer that has magnetization in a radial direction, which is suitable for perpendicular magnetic recording media.